Hi G.,
I'm curious as to why you started him on cereal before 6 months. There is increasing evidence that a baby's digestive system just can't handle solids before 6 months of age. Many feel this is why there are an increasing number of people with things like irritable bowel syndrom and such later in life.
Anyway, breastmilk or forumla needs to be the PRIMARY source of calories for at least the first year of life. Solids should compliment the milk, not supplement.
I do not feed on a schedule, I nurse my daughter on demand. Somedays she eats every 3 hours, other days she goes several hours in between feedings. (she's currently 9 months)
If you are offering cereal to your son, give the milk first. Then about an hour after milk, offer the cereal. Idealy other solids should not be started until at least 6 months of age. Also, it really is not necessary to give purees to a 6 month old. Most babies that age can handle soft cooked veggies cut into 'stick' shape. (it's easier for them to handle) The advantage of this is to introduce vegetables and eventually fruits and other foods, that the family eats. Your child is introduced to the taste and texture of the ACTUAL food instead of a watered down mushed up version of the food.
If you google 'Baby Led Weaning' you will get some good info. Also, www.kellymom.com is a GREAT resource for all mommy questions, including when and how to start solids. The main point is, milk is the primary source of calories for the first year. Solids compliment milk... not supplement.
BTW... we have done baby led weaning with our daughter. To date, she has had parsnip, celery, broccolli, zuchinni, carrot, apple, pear, cantalope, honeydew melon, watermelon, brown bread crust, white raddish, and white asparagus. She has not rejected anything. In the beginning of course not much gets 'eaten' ... but it's the experience of learning the taste and texture and allowing her to feed herself. Now, she eats most of the pieces I give her.
Good luck!